In atmosphere type furnaces for controlling the carbon content of steel or cast iron it has been a common practice for the furnace atmosphere to consist essentially of a substantially neutral carrier gas with smaller amounts of carburizing or decarburizing gases, commonly known as enriching or additive gases (typically CH.sub.4), added to shift the carbon potential of the active part of the furnace atmosphere as required for the particular process involved. These processes include gas carburization, carbonitriding, carbon restoration, clean or neutral hardening and annealing. Thus at different locations in a continuous furnace or at different times within the cycle of a batch furnace, it may be advantageous to control the atmosphere, or the atmosphere within a particular zone, at different controlled carbon potentials. In any case the steel or cast iron is heated to a controlled high temperature in a furnace containing an atmosphere that will provide carbon to the steel surface, or maintain its carbon content.
In the past a widely used source of carrier gas for furnace atmospheres was formed by thermal decomposition of a hydrocarbon such as natural gas or propane, with a limited amount of air supplied over a nickel catalyst. The resulting gas was commonly known as AGA type 302 endothermic generator gas and when natural gas was used, had a basic analysis (composition) of 20% CO, 40% H.sub.2, and 40% N.sub.2.
It is currently becoming more prevalent to use a mixture of methanol (CH.sub.3 OH) and N.sub.2 to produce the carrier gas. In the presence of heat, the methanol dissociates into one part CO and two parts H.sub.2. By adding two parts gaseous N.sub.2 a furnace atmosphere with the same basic analysis as the AGA type 302 endothermic gas, namely 20% CO., 40% H.sub.2, and 40% N.sub.2, can be produced.
In either case it has of course been recognized that the carbon potential of the furnace atmosphere must be controlled in order to provide the desired amount of carbon at the surface of the metal being treated. The carbon potential determines the ability of the atmosphere to supply, maintain or extract carbon from the surface of the steel or cast iron at the temperature to which it has been heated. This control is normally accomplished by separately controlling the analysis (composition) of the endothermic carrier gas at the generator and measuring one or more constituents of the furnace atmosphere such as, but not limited to, CO.sub.2, O.sub.2, or H.sub.2 O and making suitable adjustments in the feed rate of the additive gas or gases to provide the required carbon potential. This method is based on the assumption that the analysis (composition) of the endothermic carrier gas used remains constant and the constituents in the furnace atmosphere other than the constituent or constituents being measured also remain constant and therefore that the measured constituent or constituents represent a direct relationship to the carbon potential of the atmosphere. The latter portion of this assumption is found to be incorrect because of the well known interaction between the various constituents of the furnace atmosphere.
It was customary in the past when using AGA type 302 endothermic generator gas as the carrier gas to control the analysis of the finished gas leaving the generator by monitoring the CO.sub.2 or H.sub.2 O (dewpoint) of the finished gas and adjusting the mixture ratio of the air and hydrocarbon used to produce the finished carrier gas. When the carrier gas is supplied as a mixture of methanol and N.sub.2, the practice has been to mechanically measure or monitor the feed rate of both constituents and to adjust the ratio of methanol to N.sub.2 to provide the constant carrier gas analysis (composition) required. This procedure does not, however, compensate for such contingencies as methanol vapor or N.sub.2 bubbles in the liquid methanol supply, variations in the menthanol analysis, incomplete methanol vaporization, or mechanical inaccuracies.